This invention relates in general to fittings such as pipe connections, bulkhead doors or manhole covers for tanks and, more specifically, to a method of forming an integral internally threaded connection for a filament wound composite tank using the identical composite material to form the fitting and tank at the same time.
For aerospace applications in general, and space launch vehicle applications in particular, structural weight has the single greatest impact on vehicle performance and cost. Specifically, launch vehicles require large amounts of fuel to escape earth's gravitational forces and reach orbit. The quantity of fuel is strongly dependent upon the vehicle's structural weight and propulsion system. By decreasing overall weight and maintaining acceptable levels of structural strength and stiffness, overall vehicle performance can be greatly improved. Consequently, one effective method of evaluating a material's ability to maintain strength and decrease weight is to characterize the material by its specific strength and modulus (i.e., material strength and modulus divided by material density). High strength fiber reinforced, resin matrix composites exhibit specific strength and stiffness about 1.5 to 3 times greater than most structural metal alloys utilized in launch vehicles. Recent advances in fiber and resin technology show a trend toward continued improvement in composite specific properties.
Composite materials are typically orthotropic, which provides the very advantageous capability of tailoring the mechanical and/or physical properties of the composite for a specific structural application. This behavior is in contrast to conventional metal alloys, which generally exhibit isotropic properties. The advantages of the "material tailoring" approach are primarily realized as weight savings since designs need only use structural reinforcement (build-up) in areas and/or directions where maximum loads will occur.
Methods have been developed for manufacturing closed vessels, such as solid rocket motor casings and bottles or tanks for fluid storage, that use filament winding of resin impregnated fiber filaments, strands or tow about a mandrel. Typical mandrels are cylindrical structures with domed ends. The winding pattern, and number of layers, etc., may be selected to provide strength precisely where needed. For tanks, pipe connections are conventionally provided by positioning a metallic boss on the mandrel so that the filament windings cover and are bonded to the fitting. Disadvantages of this metal fitting approach include weight penalties, coefficient of thermal expansion (CTE) mismatches between the metal fitting and the composite tank and bonding problems between the metal fitting and the tank matrix resin.
Attempts have been made to form an internally threaded connection during filament winding without great success. In general, conventional threads in composite materials have been found to produce stress risers at the thread roots, weakening the fitting. Properly consolidating the composite material at the threads has been found to be difficult. In some cases, as described by Conley in U.S. Pat. No. 2,751,237 and Kemier et al. in U.S. Pat. No. 4,785,956 a different material than that used in the body of the vessel is used in the thread area, improving consolidation at the expense of differential CTE problems resulting from the use of non-identical materials for tank and fitting. End caps have been used at the ends of unthreaded integral fittings, as described by Thomas in Pat. No. 3,765,979, to shape the outer ends of integral fittings. However, these end caps do nothing to assure proper consolidation of the composite in these end areas. Complex, multilayer configurations have been used to attempt to increase the strength in integrally threaded areas, as described by Hill et al. in U.S. Pat. No. 4,602,480. Further, all of these patents describe sharply angled threads, which significantly increase stress at the roots and require the complex reinforcements described above.
Thus, there is a continuing need for improved methods of manufacturing filament wound composite tanks in which the tank and connection fitting are formed from the same material to eliminate differential CTE problems, stress concentrations at the fitting are avoided, proper consolidation of the composite material at the fitting is assured and a superior sealing surface is provided for a pipe connection or other fitting to be threaded into the connection fitting.